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@japanesebeginner -- please ask that as a separate question rather than cluttering the one you already have gotten an answer for. Stackexchange uses a question and answer format meaning it works best when you ask something that can be answered altogether.
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virmaiorAug 26 '14 at 0:32

My money's on onomatopoeia as well, as it's written outside of a speech bubble, like the sniffing sound くんくん and the gulping sound ごく, and it's in the same hand as those. We also see あー in this same format in the same panel as the little girl says 「あー！」.

That being said, while しょわー sounds very effervescent, the illustration makes me think it's meant to represent her shivering after drinking more of the sour beverage.

As said above, the last character before the chôon is a speedily-written わ.